1. Field of the Invention
The present invention relates to an image forming apparatus and to a method for forming an image by the image forming apparatus.
2. Description of Related Art
A related art multi-color image forming apparatus such as a multi-color electrophotographic printer includes plural process units each of which has, for example, a photoreceptor, a charging mechanism, an exposure mechanism, and a development mechanism. The related art multi-color image forming apparatus employing a tandem system, for example, includes four of such process units disposed therein. The four process units serve as image forming mechanisms of respective colors of black, yellow, magenta, and cyan, thereby sequentially transferring toner images of respective colors on a sheet being electrostatically absorbed and conveyed on the conveyance belt.
By such a related art image forming apparatus, print density may vary due to a change of sensitivity of the photoreceptor or chargeability of the toner over time or due to atmosphere temperature or humidity therein. Consequently, the print density is detected at a prescribed timing at which, for example, a power source of the image forming apparatus is activated or a prescribed number of sheets are printed, so as to perform the density correction.
In such a related art image forming apparatus, a density detection pattern used for the density correction is printed on the conveyance belt, and density of the density detection pattern is read by a density detection mechanism. According to a result provided by the density detection mechanism, a physical characteristic (e.g., development voltage, exposure time) of an engine unit of the image forming apparatus is adjusted, so that the density correction is performed, thereby enhancing stability of the print density. Such a density correction is disclosed in Japanese Un-examined Patent Application Publication No. 2004-258281, for example.
Moreover, the density detection pattern is printed on the conveyance belt, and the density of the density detection pattern is detected by the density detection mechanism in a state that the above density correction result is added. A density value detected by the density detection mechanism is notified to an image processing unit of the image forming apparatus. The image processing unit corrects the density based on a difference between the density value notified and a target density value (such a correction is hereafter referred to as a gradation correction), thereby enhancing stability of the print density. In such an image forming apparatus, the density correction process is executed by correcting the physical characteristic of the engine unit thereof and performing the gradation correction by the image processing unit based on the correction result of the physical characteristic.
In a mechanism adjusting the physical characteristic of the engine unit of such an image forming apparatus, for example, each of a voltage correction adjusting development voltage and a light amount correction adjusting an exposure time and the like of an exposure device is executed. When one of such corrections is completed, the density detection pattern is again outputted and detected to perform another one of the corrections, causing prolongation of the time in an amount of outputting and detecting the density detection pattern plural times.